Yes, you can follow the instructions for programming OATH-HOTP for KeePass (although I personally recommend going the HMAC-SHA1 Challenge-Response route instead -
http://www.kahusecurity.com/2014/securi ... nd-factor/)
Gmail is easy enough, assuming you're using Google Chrome, you can register as many Google accounts as you want from a single YubiKey NEO. Gmail uses U2F which will not affect your ability to program an OATH-HOTP or Challenge-Response credential with the YubiKey Personalization Tool.